Abstract Leptospirosis is the most widespread zoonotic disease worldwide, and continues to emerge as a significant infectious disease in urban slums, particularly in tropical regions. Several species of the genus Leptospira can cause infection, which varies in severity from mild illness to fatal hemorrhagic disease with multiple organ failure. Typically, infection of a maintenance (reservoir) host is chronic and asymptomatic. Yet, the same bacterium can cause acute, potentially life-threatening infection in an accidental host species. Persistence of leptospires in wildlife, companion animals, and livestock provides a constant reservoir for human infection, as infected animals harbor Leptospira in the proximal tubules of the kidney and excrete Leptospira in the urine. Release of viable bacteria into the environment provides opportunities for infection of new hosts. Infection is acquired through exposure to animal bodily fluids, especially urine, or from environmental sources contaminated with urine. Entry is typically through mucous membranes or minor breaches in the skin, followed by dissemination to multiple sites. Widespread endothelial damage is a prominent feature of leptospirosis. Despite the global burden of disease, the pathogenic mechanisms of Leptospira species are understudied. Adhesion to host molecules is critical to the abilities of many pathogens to establish infection and cause disease, and is likely true for Leptospira, as well. Adhesion to host cell surface molecules may facilitate invasion of tissues, but to date this question has not been mechanistically addressed. While several candidate adhesive proteins have been identified, how these activities contribute to pathogenicity in vivo remains unknown. We demonstrated that the major contributor to endothelial cell-cell junctions, VE-cadherin, is a receptor for L. interrogans. We also identified two adhesins that bind to VE-cadherin, demonstrated that L. interrogans and purified recombinant adhesins cause disruption of adherens junctions, which are critical to maintenance of endothelial integrity. In our work on another spirochete, Borrelia burgdorferi, we developed a short-term intravenous inoculation model to determine the roles of B. burgdorferi proteins in interactions with the vasculature in different tissue sites in living mice. In this exploratory/developmental project, we will test the hypothesis that specific L. interrogans proteins contribute to bacterial interaction with the endothelium in vivo. In Aim 1 we will test the hypothesis that L. interrogans interacts with the vascular endothelium in living animals, and in Aim 2 we will test the hypothesis that specific L. interrogans proteins that adhere to VE-cadherin in vitro have roles in adherence of the bacteria to the endothelium in vivo. This exploratory/developmental work will refine the mechanism by which three L. interrogans adhesins contribute to the pathogenesis of leptospirosis.